The known methods and apparatii for oxygen delignification of medium consistency pulp slurry consist of the use of high shear mixers and single reactors with retention times of twenty to sixty minutes. These are operated at consistencies of ten to fourteen percent (o.d.) at an alkaline pH of from 10 to 12.5. Oxygen gas and hydrogen peroxide are contacted with the pulp slurry in a turbulent state lasting less than one second. The oxygen gas and hydrogen peroxide are both added prior to the high shear mixer, either simultaneously, or the hydrogen peroxide is added prior to the oxygen by 10-300 seconds. To date, sulfite pulp systems of the aforementioned design have resulted in 60-70% Kappa number reduction and a brightness increase of 20-25% ISO. It has been reported that over half of the Kappa number reduction can occur at the high shear mixer, after the oxygen gas is introduced. Final brightness of 84-86% ISO can be achieved with additional hydrogen peroxide bleaching steps.
The disadvantages of the known methods is that high total dosages of hydrogen peroxide, often in excess of 5.0% are required to achieve a mid-80's ISO brightness, and this often requires two separate hydrogen peroxide bleaching stages following the oxygen delignification stage.
It is understood that oxygen delignification reaction proceeds under two distinct orders of reaction kinetics. The first reaction occurs rapidly, and is responsible for lignin fragmentation (delignification). It is a radical bleaching reaction that is dependent on alkali concentration or pH to proceed. It also consumes alkali (e.g., NaOH) as it proceeds and generates organic acids, causing pH to drop by one-half to one point. This is consistent with prior noted field observations. The second reaction occurs slowly, at a rate estimated to be twenty times slower than the first reaction. This reaction is responsible for the destruction of chromophoric structures (brightness development). It is an ionic bleaching reaction that is dependent on alkali concentration, and pH, to proceed. It also will consume alkali as it proceeds and generate organic acids, causing the pH to drop by one to two points during the reaction time.
The addition of hydrogen peroxide (H.sub.2 O.sub.2) to an oxygen delignification stage will increase both orders of the reaction kinetics, resulting in increased delignification and brightness. It will, for sulfite pulps, have the largest impact on the first rapid, delignification reaction. The impact of the peroxide slows dramatically during the second brightening reaction. This may be due to the applied hydrogen peroxide reacting as both a delignification and a brightening agent in the first reaction. This will consume hydrogen peroxide and increase alkali consumption during the first order reaction. Corrections in hydrogen peroxide and alkali will be required for the second reaction to proceed efficiently.